The invention relates to a method for the manufacture of a composite casting from an aluminium alloy and a wear-resistant material, the latter being applied onto a core, which can be detached from the finished compound casting and where, after immersion in an aluminium melt, which is necessary for the formation of an intermetallically combined, thin aluminium coating, the aluminium alloy is cast around the material in a pressureless manner.
It is known for example from German Pat. No. 1,291,865, to spray a layer of steel on a core and then to cast aluminium around this layer by the die-casting method. The connection between the aluminium and the steel layer occurs in a purely mechanical manner. When cooling down, the solidifying aluminium shrinks onto the coated core and is clamped, during this process, with the rough sprayed layer. Therefore, this method is only successful if the surface of the layer applied to the core has a sufficient roughness and if the aluminium is introduced into the casting mould under great pressure. For these reasons, the method is dependent on rough layers and is confined, for the rest, to the application of aluminium layers by the die-casting method. However, these limitations are decidedly a hindrance in some cases of use. For example, light-metal cylinders for internal combustion engines are manufactured predominantly by the low-pressure casting and chill casting methods. However, for these casting methods, too, ways have already been found for the manufacture of the abovementioned compound castings. In this connection, use has been made of the realization that intermetallic compound layers may form between superimposed layers of aluminium and steel, at the boundary surfaces thereof. A relatively strong adhesive bond between the superimposed layers can be brought about by these compound layers. The manufacture of such compound castings is described in detail, for example, in German Pat. No. 860,303 or German Pat. No. 971,052. These methods have also become known under the title A1-Fin method.
However, satisfactory adhesive strengths can only be achieved with this method of coating under very specific conditions, and because of these its application is again only possible within corresponding limits. For example, it is no longer possible to bring about a secure connection if oxides are present on one of the adjacent boundary surfaces. For, in this case, due to the very great affinity of aluminium relative to oxygen, the desired metallic compound is virtually prevented from the outset by the formation of an aluminium oxide layer. Therefore, the A1-Fin methods are performed in such a way that the part which is to be coated with aluminium is, first of all, immersed in an aluminium bath for a short time. In this manner, one obtains a thin aluminium coating which is intermetallically combined with the steel part that is to be coated. Due to the immersion of the part that is to be coated in a liquid aluminium melt, it is almost impossible for the aluminium to combine with the oxygen of the air on the surface that is to be coated. Attention has to be paid, too, that there is not any free or combined oxygen present in the surface of the part that is to be coated, which might lead to the formation of an oxide layer in the aluminium that is to be applied.
To this extent, it seems that parts with rough and porous surfaces, as found particularly on thermically sprayed metal layers, have hitherto been uncoatable by the A1-Fin method. After all, the oxide content in metal spray layers is easily up to 60 %. Of course, theoretically it would be possible to eliminate to a very large extent the problems caused by oxidation by working under a vacuum or in a protective gas atmosphere, that is to say in this case they would not arise in the first place. However, this would also apply to metal spraying and the introduction of the sprayed layer into the aluminium bath. However, for economic reasons alone, it is not possible to take this course of action in practice.
According to the prior art, it has therefore hitherto only been possible to keep the surfaces of the parts to be coated with aluminium free from oxide and then to coat them by the A1-Fin method or else to apply the aluminium layer by the die-casting method, in which case one had to be content with a mechanical bonding of the adjoining materials. It is a great disadvantage that the application of the A1-Fin method is confined to parts which have entirely or at least substantially oxide-free surfaces. Attempts have been made to eliminate this disadvantage partly be subsequently removing the surfaces containing oxides, which can be done for example by mechanical methods such as sand rays or other material removing treatment processes.
Very special problems, which can usually hardly be solved, emerge in this respect in the case of layers which are manufactured by the metal spraying method and which have to be coated with aluminium according to the A1-Fin method, because of the extremely high oxide content in such layers.